This invention relates to blends comprising a polyphthalamide, preferably a crystallizable or crystalline polyphthalamide, and more particularly to blends comprising polyphthalamide, a functionalized rubbery modifier, a functionalized polyolefin and polyolefin having improved tensile properties and rigidity together with good ductility, and to a method for improving the ductility of polyphthalamides.
The highly desirable balance of strength, stiffness and thermal properties of polyamides has gained these resins wide acceptance in the molding arts and in the extrusion arts including fiber spinning and film extrusion. The resins are found in a great variety of applications, and may be particularly attractive for use where resistance to chemical and thermal attack is required. Crystalline polyamides comprising aliphatic terephthalamide units exhibit particularly good thermal, strength and stiffness properties, and low water absorption relative to nylon 6,6. Crystalline and crystallizable polyphthalamide resins are known in the art, and a variety of grades are readily available from commercial sources, including for example the Amodel.RTM. Polyphthalamides from Amoco Performance Products, Inc. Such resins have relatively high melting points, generally about 290.degree. C. or higher, and the temperature of onset of thermal degradation for many does not greatly exceed their melting points, making the thermal processing of these resins a more rigorous and complex task, particularly for filled and reinforced polyphthalamides, than is encountered for lower melting polyamides including nylon 6,6 melting at about 260.degree.-265.degree. C.
Polyphthalamides are typically low in ductility as reflected by generally low values for elongation at break and the lack of significant extensibility in tensile testing. In a dry, as-molded condition, such resins also may be deficient in impact properties as measured by mechanical tests such as the Notched Izod Impact Test or the high speed instrumented Dart impact Test. Articles molded from such highly rigid, low ductility resins may be subject to cracking, and to brittle failure in stress. Where it is expected that a molded part will be subjected to repeated flexings, or where the part is required to be rigid and to exhibit a spring-like behavior when flexed, lack of a reasonable level of ductility may lead to premature failure of the part.
Considerable effort has been made over the years to improve mechanical properties of polyamides. Plasticizing additives are available to improve the flow and ductility of nylons, and a variety of polymeric additives, generally rubbery compositions that can be blended or incorporated into the polyamides, have been developed for improving ductility and impact strength. Of course, it is important that such improvements be achieved without substantial adverse effects on particular properties of the polyamide needed for use in the intended application. It also is important that the additives retain their effectiveness after processing of polyamide compositions containing the same and during use of articles prepared therefrom.
A number of additives are known for use in improving the impact strength of aliphatic nylons and their melt processability. Generally, the additives are rubbery compositions that can be blended or incorporated into the polyamides to provide improvement in impact strength. For example, the use of from about 10-50 wt. % of a pendant succinic anhydride group-containing reaction product of maleic anhydride with a hydrogenated polymer of a conjugated diene or hydrogenated random or block copolymer of a conjugated diene and a vinyl aromatic hydrocarbon with a variety of polyamides is disclosed in U.S. Pat. Nos. 4,427,828, issued Jan. 24, 1984, and 4,508,874, issued Apr. 2, 1985, both to Hergenrother et al. Impact resistant blends of a polyamide and a thermally stable, modified, selectively hydrogenated, block copolymer of a conjugated diene and a vinyl aromatic compound are disclosed in U.S. Pat. No. 4,783,503, issued Nov. 8, 1988, to Gergen et al. Olefin copolymers containing acid groups are disclosed for use with polyamides in British Patent Specification 998,439 to E. I. dupont de Nemours and Company, and U.S. Pat. No. 4,174,358 to Epstein, issued Nov. 13, 1979, discloses polyamides having a polymer dispersed therein as discrete particles 0.01 to 1 micron in size, the dispersed polymer having sites that adhere to the polyamide.
Rubbery modifiers may also be used in filled polyamides. U.S. Pat. No. 4,537,929 to Nangrani, issued Aug. 27, 1985, discloses maleated ethylene copolymer impact modifiers for use in producing high impact neat resin compositions comprising an aliphatic nylon. The impact modifiers are shown to be less effective in improving the impact of glass fiber-reinforced formulations. U.S. Pat. No. 4,659,752 to Piret, issued Apr. 21, 1987, further describes the difficulty of improving the impact properties of glass reinforced thermoplastics. According to patentee, the level of rubber needed to impart a significant improvement in the impact properties of filled resins has a marked detrimental effect on other mechanical properties.
Combinations of impact modifiers have also been employed. For example, in U.S. Pat. No. 4,849,471 to Saito et al., issued Jul. 18, 1989, patentees employed a mixture of hydrogenated block copolymers and functionalized ethylene-containing polymers in a variety of neat polyamide compositions including a 50/50 copolymer of hexamethylene polyterephthalamide and hexamethylene polyisophthalamide. U.S. Pat. No. 5,122,569 to Scheibelhoffer et al., describes the use of peroxide-coupled combinations of elastomers and functionalized polymers as modifiers for polyamides. U.S. Pat. No. 4,795,782 to Lutz et al., issued Jan. 3, 1989, discloses the use of hydrogenated block copolymers, functionalized hydrogenated block copolymers, polypropylene and functionalized polypropylene in various combinations as modifiers for aliphatic nylons.
Although the modifiers described in the prior art are effective in improving impact when used with aliphatic nylons, generally a high level of modifier, often as much as 20 wt % or more, is needed in order to attain desirably high impact properties. The presence of rubbery modifiers at these high levels has a marked effect on other important mechanical properties, generally reducing the rigidity of impact modified aliphatic nylon blends as reflected in the modulus of the resin formulation, as well as tensile properties.
The high level of rigidity and good tensile strength characteristics of polyphthalamides are very important to many applications. Overcoming the low ductility of high temperature polyphthalamides while maintaining the rigidity to a high degree, even at elevated temperatures, would widen the commercial utility of these polyphthalamides. The prior art rubbery modifiers conventionally employed in compounding high impact aliphatic nylons markedly reduce rigidity and strength properties when used at levels needed to achieve significant impact improvement. Similar reductions in rigidity for polyphthalamides would reduce their attractiveness for many applications. Modifiers capable of improving the ductility and particularly the impact characteristics of molded polyphthalamide articles and extruded goods when used at low concentration levels and, further, capable of retaining good ductility over a wide range of temperatures, would give compounders and resin formulators greater flexibility in tailoring the property balance of polyphthalamide blends to minimize the loss in rigidity and tensile properties and provide materials to suit a wider variety of applications.